Pit records on how to call child component methods in the parent component of the vue3.0 project

In the process of project development, it is inevitable to call the method in the child component in the parent component, using the following

in vue2 project

// 父组件
<Children ref="child"></Children>

<script>
	mounted() {
    
    
		this.$refs.child.XXX() // XX为子组件的一个方法名
	}
</script>

in vue3 project

// 父组件
<Children ref="child"></Children>
<script>
  import {
    
     onMounted, ref } from 'vue'
	setup() {
    
    
		const child = ref()// child要和子组件的ref值一致
		onMounted(() => {
    
    
			child.value.XX() // XX为子组件的一个方法名
		})
		return {
    
    
			child// 此处一定要return，否则获取到的child.value是undefined
		}
	}
</script>